1. Field of the Invention
The present invention relates to an anode active material for a lithium secondary battery and its manufacturing method, and more particularly to an anode active material for a lithium secondary battery and its manufacturing method that may improve an initial capacity and a reversible capacity by doping metal nitride or silicon nitride with a metal element such as Co and Fe.
2. Description of the Prior Art
At present, a lithium ion battery is widely used as a power source of mobile equipments, and attracts interests as a next-generation power source for an electric automobile. As the use of mobile equipments is abruptly increased, many devices with various functions desired by users are unceasingly developed, so that energy consumption of these mobile devices is also increased.
The lithium ion battery has an anode made of graphite, a cathode made of lithium transition metal oxide, a separator, and an organic solvent electrolyte in which lithium ions are dissociated. In order to develop a lithium ion battery with high energy, a material that has higher energy density than an existing electrode material should be used.
Studies for anode active materials are briefly classified into oxides, suicides, lithium alloys, and nitrides. Study for oxides is mainly focused on elements with 3d orbit, representatively cobalt (Co) oxides (CoO, Co3O4) and ferrum (Fe) oxides (a-Fe2O3). When Si and Sn are reacted with lithium, an irreversible lithium oxide (Li2O) is generated, while it is reported that the cobalt oxides are reversible oxides. Study for suicides uses ternary alloy reaction between metal silicide and lithium, representatively magnesium silicide (Mg2Si) and nickel silicide (NiSi). Such suicides show behavior similar to graphite, but they show serious lattice distortion according to complexity of the alloying process. Si and Sn representatively use a binary alloy reaction of metal and lithium. In particular, silicon, when being made into an alloy in the form of Li4.4Si by reaction with lithium ions, has a theoretical capacity density of 4,200 mAh/g, which is higher than that of graphite, 370 mAh/g, and silicon also shows charging/discharging voltage behavior similar to graphite. However, they are not commercially used yet due to problems such as repeated contraction/expansion of lithium silicide (LixSi) in charging/discharging and resultant poor electric contact between active material and a collector, like silicides. Study for nitrides is mainly focused on the ternary system using lithium nitride (Li3N) and the binary system such as tin, zinc and copper nitrides. If lithium nitride is doped with cobalt, an irreversible capacity is hardly generated at an initial cycle and a constant capacity of 800 mAh/g is obtained. However, there are many difficulties in commercializing them, since lithium nitrides show high reactivity with atmosphere and sensitivity with moisture. Study for tin, zinc and copper nitrides is mainly focused on electrodes in a flat film shape.
In spite of many studies for various materials as mentioned above, commercialization is not yet realized due to various problems. In particular, nitrides are not commercialized in view of economical efficiency for installation and facility costs due to reactivity with atmosphere and moisture despite of their excellent features.